Evacuated device and method of exhaust



Nov. 15, 192 7.

1,648,958 R. E. MYERS EVACUATED DEVICE AND METHOD OF EXHAUST Filed May 3, 1923 INVENTOR RMP/l E. MYERs BY ATTORNEY Patented Nov. 15, 1927.

UNITED STATES 1,648,958 PATENT OFFICE.

RALPH EMERSON MYERS, OF EAST ORANGE, NEW JERSEY, ASSIGNOB TO WESTING- HOUSE LAMP COMPANY, A CORPORATION OF PENNSYLVANIA.

'EVAGUATED DEVICE AND METHOD OF EXHAUST.

Application filed May 3,

This invention relates to evacuated devices and methods of exhausting the same and more particularly to the cleaning up of residual and occluded gases in electron devices, containing oxide-coated filaments, by the use of magnesium or the like.

An object of my invention is the employment of magnesium in three-electrode tubes using oxide-coated electron-emittin filaments to simplify the exhaust so iedule thereof and, at the same time, give an increased electron-emission to the filament.

Another object of my invention is the manufacture of electron devices, containing oxide-coated cathodes, in such a man- ,ncr that moreuniform results and improved emission are effected.

A further object of my invention is the improvement in the manufacture of electron devices, including refractory-metal wire coated with oxides of the alkaline earths, asthe cathodes thereof, by using a clean-up agent, such as magnesium, which vaporize: at such a low temperature that it is feasible to employ a cheaper and yet better plate material such as nickel, instead of molybdenum which was formerly required when other clean-up materials, vaporizing at higher temperatures, were employed.

A still further object of my invention is the employment of a clean-up agent in evacuated devices, especially electron-devices with oxide-coated filamentary cathodes,

which not only is effective when first vaporized from the plate, but which, after vaporization and deposition on the bulb, is still effective as an absorbent ofresidual gases.

Other objects and advantages of the invention will becomeapparent as the description proceeds. In the manufacture of electron tubes containing cathodes or filaments made of platinum-iridium coated with oxides of suitable elements of Group II of the periodic system or alkaline earths, for example, a mixture of the oxides of barium and strontium, it has been found necessary to exhaust, to a very low pressure, using mercury diffusion pumps combined with a complicated electrode treatment during the exhaust period. The aforementioned operation is very slow and expensive and rather uncertain in its results. In the copending application of H. C. Rentschler Serial No. 582,261, filed August 16, 1922, vacuum devices and method of ex- 1923. Serial No. 636,510.

hausting the same and assigned to the Westinghouse Lamp Company, is disclosed a method of evacuating electron devices by using aluminum, with or without some other material such as magnesium, calcium or phosphorus, after a moderate exhaustion thereof. It was thought necessary touse a metal of fairly high vaporization point,

such as aluminum, so that at least some of it would remain on the plate after a sufiicient heat treatment thereof to remove substantially all occluded gases. That is, it was thought that after the metallic clean-up agent was once vaporized and deposited on the inner wall of the bulb, that it was no longer effective for cleaning-up any residual gases and that any occluded gases driven off after the metal was all deposited on the bulb would remain therein as residual gases, because there was no provision for removing the same. For this reason, aluminum was used alone or in combination with other materials so that this clean-up action, upon the initial vaporization thereof, would not be completed until the plate had been properly heat-treated.

When using aluminum attached to the plate as a clean-up agent, it is necessary to make the plate of a refractory material, such as molybdenum, because of the high temperature to which it must be subjected to fully vaporize the aluminum. Besides this disadvantage when using aluminum, as will be obvious, any aluminum oxide formed will be inert with respect to carbon dioxide, which is one of the gases which it is desired to eliminate.

Phosphorus, which is a well known cleanup agent, is not suitable in an electron device in which an oxide-coated cathode is employed, because it deleteriously affects the electron-emitting oxides and reduces the emission thereof. It also, if used in suflicient quantity, harmfully affects the platinum alloy support, if such is used to carry the oxides, rendering it brittle and subject to breakage.

According tomy invention, I use magnesium or its equivalent, which does not deleteriously aifect the alkaline earths, for or ample, the oxides of barium and strontium, which are commonly used as electron-emission material, and attach it to the plate of an electron device. In this instance, it is not necessary to make the plate 'of molyb- .denum, because magnesium is vaporized and activated at such a low temperature that nickel or other cheaper plate material may be used for the purpose.

Magnesium may be attached to the plate in any desired manner and the plate together with a suitable grid and an oxide-coated filament may be suitably mounted and inserted in a bulb. The mount may then be sealed into the bulb and the bulb exhausted by ordinary methods, although it is obvious that' the better the'vacuum obtained, the less residual gas to be cleaned up and, there fore, the smaller the quantityof magnesium that need be applied to the plate.

After exhausting, the bulb is tipped off and is then ready to have the magnesium activated or vaporized therein. This is preferably accomplished by high-frequency induction in the manner disclosed with respect to aluminum in the aforementioned application of Rentschler, although it is not necessary to heat the plate to as high a temperature, since the magnesium vaporizes at a lower temperature than aluminum.

It will be found that the excess magnesium over that used in absorbing gases in the bulb and compounds of magnesium formed will deposit as a film or mirror on the inside of the bulb. If it be assumed that the coating on the filament is pure alkaline-earth material, that is, for example, a mixture of the ure oxides of barium and strontium, no

urther clean-up is required of the magnesium. However, as preferably employed, the electron-emitting filament may be coated with carbonates of the alkaline earth metals, instead of with oxides thereof.

A method of preparing oxide-coated filamentary material, by first applying carbonates thereto and su sequently decomposing the carbonates to drive off carbon dioxide and leave the desired oxides thereon, is disclosed in the eopending application of D. MacRae, Serial No. 587 ,270, filed September 11, 1922, electron-emitting devices and method of making and assigned to the West inghouse Lamp Company. the electron-emitting filament is coated with a mixture of the carbonates of barium and strontium, it will then be necessary to heat it to drive off carbon dioxide therefrom and leave the desired oxide mixture thereon. Such carbon dioxide driven off upon heating the filament may be removed or absorbed by the film of magnesium, and compounds thereof, such as the oxide deposited on the bulb during the preliminary cleaning up operation.

A preferred method for effecting this secondary clean-up action is to season or burn the filament for a short length of time while electrically connected to the grid and with a fairly-high voltage. established with respect to the plate. This procedure apparently Assuming that causes the carbon dioxide, which was evolved from the filament, to become ionized and thereby preconditioned for being effectively cleaned up or absorbed by'the film of magnesium and its oxide on the bulb.

Although I have described the clean-up action of magnesium after tipping off the bulb, it should be understood that the magnesium may be activated, although less conveniently, while the device is still on the pumps in which case the surplus magnesium and magnesium compounds, if any, will deposit on the glass bulb in the same manner as heretofore.

My invention will better be understood by reference to the accompanying drawing in which the single figure illustrates, partly diagrammatically, an electron tube and apparatus for treating the same in accordance with my invention together with the following description of a preferred method of practicing the same.

The oxide-coated filament 1 for the electron device 2 is preferably prepared by first coating a platinum-iridium ribbon or wire of the desired size with a mixture of the carbonates of barium and strontium, according to the method disclosed in the aforementioned application of MaeRae. Briefly, the method comprises making an aqueous suspension of a mixture of pure powdered barium carbonate and pure powdered strontium carbonate in approximately the proportions of 9 grams of the former mixed with 10 grams of the latter and about 18 c. c. of water added gradually to form a suspension of uniform consistency. The suspension is then preferably applied uniformly to the ribbon or wire by means of a gettering wheel and the wire drawn slowly through an electric tube furnace, where it is heated in an atmosphere of carbon dioxide to bake the coating firmly thereto without decomposing the carbonates. The coated wire formed will be stable in air and may be mounted adjacent a suitable grid and plate for introduction into a bulb to make an electron device.

The plate 3 may be formed of molybdenum, but when using magnesium as a cleanup material according to my invention, it is preferable to make it of nickel because the same is more economical, and possibly acts to a slight degree as a clean-up agent when heated. The grid 4 may be made of material similar to that from which the plate is made. Before insertin and sealing into the bulb 5, there is prefera ly attached to the plate, a small piece of magnesium foil 6 or ribbon which when used for a small size tube need not weigh more than about a milligram. The magnesium may be attached to the plate in any desired manner as by welding or crimping. After sealing a mount 7 formed of the aforementioned plate, grid and filament into a bulb, the bulb is preferably exresidual atmosp referred manner to leave 9. here at a pressure of about from 1 to 15 microns. .The better the vacuum obtained, the less the amount of magnesium required, although enough should be used to clean-up the gases evolved from the plate and filament.

It is not necessary to exhaust to as hlgh a degree when using magnesium as a cleanup agent as when using no clean-up materlal and better and more uniform results are obtained besides. The bulb may then be sealed off or the ma vated, preferably by high frequency induction in the manner disclosed in the aforementioned Rentschler application, while the bulb is still on the pump. a

After sealing off the bulb 5 and heating the plate 3 to vaporize the magnesiunrfi, drive off occluded gases and clean up residual and occluded gases by means of the vaporized magnesium, it is then time to heat the filament 1 to change the carbonates thereon into oxides, so as to develop the emissivity thereof. This may be done by passing a current of sufficient strength through the filament, for example, from battery 8 to heat it to the decomposing temperature of the carbonates thereon. The battery 8 may be connected to filament leads 9 and 11 through a switch 12 and variable resistance 13, as will be readily understood by those skilled'in the art. Thus, the filament may be seasoned for about a half hour by being heated to a bright red heat and at the same time, it is connected, preferably at its negative end, to the grid 4 by lead 14, as shown, and a potential difference of from 60 to 90 volts maintained between it and the plate, by means of battery 15 connected, as shown, preferably through resistance 16 and switch 17 to leads 18 and 11. The resistance 16 may. if desired, take the form of a 10 watt tungsten filament lamp. The carbon dioxide driven off from the filament 2 will become ionized and probably thereby better conditioned for being cleaned-up or absorbed by the film or mirror of magnesium and its oxide on the inside of the bulb. The reactions for cleaning up the carbon dioxide are thought to be as follows:

Because of the space-discharge through the carbon dioxide, upon ionization thereof, during the foregoing treating process, a portion of said carbon dioxide is converted to carbon monoxide, as it is a known fact that an electrical discharge in carbon dioxide produces some carbon monoxide. It is also of some interest to note that an electrical discharge in carbon monoxide produces some carbon dioxide, so that in either case, a mixture of carbon monoxide and carbon dioxide hausted in any esium may be actiresults, accordin to the following references from theliterature (See Applied Electro-Chemistry, A. J. Allmand 1920, pages 190 and 191; research staff of General Electrical Company, London; Transactions of the American Electrochemical Society, 42, 1923; Alfred Holt, J r., Journal of the Chemical Society, 95, 30-4, 1909; A. Moser and N. Igarishev, Z. Elektrochem. 16, 613-20, 1910.) The oxygen liberated according to the equation, CO =CO+O, is cleaned up by the film of magnesium on the bulb, according to the equation The mixture of gases produced, is not cleaned up immediately by the magnesium and remains in the bulb for a short time, where it exerts a beneficial effect on the emissivity of the filament, probably because of the electron-discharge from the filament, through said mixture. Eventually, however, on continuance of the electron-discharge, the gases are removed, as indicated by the reduction of the grid current to a negligible quantity. This may take place in accordance with the equation, 2CO=CO +C, the CO in the mixture and that being formed being cleaned up by magnesium oxide formed, according to the corresponding equation before set forth or being again dissociated into CO and O and these cleaned up as described above.

It has been found that by using the aforedescribed method of cleaning up residual gases, that not only is a very good vacuum obtained in the device, but an improved emissivity of the oxide-coated filament is effected and more uniform results are obtained. It seems, that by using only a metal which does not deleteriously affect the oxidecoating and which, on the other hand, efficiently cleans-up the carbon dioxide' evolved when the carbonates are broken downto,

oxides, that such oxides are very much more efficient than when such an electron device, was prepared according to former methods.

Although I have described what I now consider the preferred method of practicing my invention, it is to be understood that the same is merely illustrative and that my invention is only limited by the spirit and scope of the appended claims.

What is claimed is:

1. The method of manufacturing an electron discharge device having an electronemitting cathode which comprises coating the cathode with the carbonates of alkaline earth metals, incorporating the cathode in the device, exhausting the device, heating the cathode to convert the carbonates of the alkaline earth metals to the oxide thereof, ionizing the oxides of carbon liberated by the cathode and subsequently removing the gaseous. content of the tube to create a vacuum therein.

2. The method of manufacturing an electron discharge device having an electronemitting cathode which comprises coating the cathode with the carbonates of alkaline earth metals, sealing the cathode into the envelope of the device, exhausting the envelope, heating the cathode to convert the carbonates into the oxides of the alkaline earth metals and to liberate oxides of carbon in the envelope, passing a discharge through said oxides of carbon to ionize the same and subsequently removing the oxides J the envelope, heating the cathode to convert the carbonates to the oxides of the alkaline earth metals with the liberation of oxides of carbon, heating the anode to vaporize the metallic clean-up agent thereon and drive ofi occluded gases therefrom, ionizing the gases liberated by the cathode and cleaning them up with the metallic clean-up agent.

4;. The method of producing an electron discharge device employing an electronemitting cathode and a plate electrode comprising applying to the cathode a carbonate of an alkaline earth metal, attaching a quantity of magnesium to the plate electrode, incorporating the cathode and plate elec trode into the envelope of the device, exhausting the envelope, heating the plate electrode to drive out the gases therefrom and vaporize the magnesium, heating the cathode to convert the carbonates to the oxide and passing a discharge between the cathode and plate electrode to ionize the gas in the envelope.

5. The met-110d of manufacturing an electron discharge device having an oxide coated cathode and cooperating anode comprising incorporating into the device the anode with a quantity of magnesium attached thereto and the cathode coated with carbonates of alkaline earth metals, evacuating the envelope, heating the anode to drive out 00'- cluded gases therefrom and vaporize the magnesium thereon to form a film of magnesium on the envelope of the device and heating the cathode to drive oii oxides of carbon therefrom and maintaining an ionizing influence in the envelope to improve the emission of the cathode.

(i. The method of making an electron device having an oxide coated cathode and an anode comprising coating a metallic filament with a mixture of the carbonates of barium and strontium, attaching a piece of magnesium to the anode, sealin the cathode and anode into a bulb, exhausting the bulb, heating the anode to drive 01f occluded gases, Va porize the magnesium and cause some of it to deposit as a film on the inside of a bulb and clean up residual and driven-01f gases, heating the filament to decompose the carbonates thereon and leave electron-emitting oxides with a liberation of carbon dioxide and ionizing the carbon dioxide, to cause the same to decompose partially with the generation of carbon monoxide, whereby the electron-emissivity of the filament is improved.

In testimony whereof, I have hereunto subscribed my name this 2nd day of May, 1923.

RALPH EMERSON MYERS. 

